G83 covered generation up to 16A per phase, about 3.6kW, so if you wanted to pop a 5kW generation facility on a single phase supply you'd technically go beyond G83 into G59 territory.

However, if you're just over the limit, many DNO's will allow an extended G83 connection but may charge for this or insist upon being present when your installer makes the final connection. If they insist on G59 then this requires prior authorisation.

G59 covers up to MW+ scale installations - in which case you would certainly need to do a full upstream network analysis via the DNO, at your expense. I'm assuming though you're talking microgen scale?
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Quisling